In recent years, 3D techniques have been greatly advanced. Various applications have been developed for a 3D display which is one of output devices giving the most sense of immersion. A 3D display of a stereoscopic manner using two images (or photographs) is widely used in a movie theater and as a home TV. In recent years, the 3D display is combined with UHD so that the reality is enhanced and HMD is widely distributed due to the development of a small size display. Thus, it is expected that techniques related to the 3D display is consistently developed and the market will be more grown in future.
In this situation, DIBR has taken an important role. The DIBR is a scheme that generates an image having several viewpoints by using a central image and a depth image. This scheme has merits which can easily store and transmit the image due to a small size as compared with that of a stereoscopic scheme having an image of all viewpoints, and can control the value of a depth factor (baseline distance) for user convenience. Due to those merits, the DIBR is variously utilized for a multi-view which gives various viewpoints, auto-stereoscopy which enables a user to see a 3D image without any 3D glasses, a 2D-to-3D conversion which converts a previously taken 2D image into a 3D image, etc.
As techniques related to DIBR are developed, there has risen a need to provide a technique of protecting copyright of DIBR contents. Although there is a watermarking scheme that is the most typical scheme of protecting contents, it is difficult to apply the watermarking scheme according to the related art used for a 2D image to the DIBR technique.
For example, a content user may generate several synthesized images having new viewpoints based on an image provided from a content provider.
In such a DIBR system, since the content provider can insert a watermark only into a central image and cannot insert a watermark into a synthesized image newly created by a content user, the DIBR may treat the image as one attack on the central image.
However, since the DIBR is a non-linear geometric distortion in which all pixels travel mutually different distances in a horizontal direction, when a 2D watermark of the related art is used, it is difficult to detect the watermark so that it is difficult to protect the copyright of an image synthesized by a user.
To solve the problem, there have been proposed several DIBR watermarking techniques.
As one example, a watermark is inserted by previously predicting a degree of a movement of a central image pixel by using a depth image. Thus, three watermarks, that is, central image watermark and left eye and right eye image watermarks are inserted into the central image. In this case, it is easy to detect a watermark even after DIBR and, even after a JPEG noise addition which is the most common distortion, the high detecting rate may be obtained.
However, since a degree of a movement of a pixel is predicted by using a depth map, the watermark is modified due to a depth map preprocessing and is weak when the depth factor value is changed. In addition, the technique is not suitable to be used for Multiview video plus depth (MVD) in which several viewpoints are generated and the 2D-to-3D conversion in which any depth maps do not exist.
As another example of the technique, a watermark is inserted by applying a quantization scheme to a dual-tree complex wavelet transform (DT-CWT) domain. This scheme uses a DT-CWT coefficient having the smallest directional variation among DT-CWT coefficients after DIBR transformation. In addition, the technique is designed to perform the quantization in units of row by using the property of the DIBR in which only a horizontal movement exists, so that the technique is robust against DIBR. Since the technique is also robust against the depth map preprocessing and the baseline distance adjustment as well as the DIBR and the JPEG compression and size scaling, the technique is suitable to be used for the MVD and the 2D-to-3D conversion.
However, the number of inserted bits is limited, and the technique is vulnerable to a noise adding and a geometric distortion such as a cropping.
As still another example of the technique, there is a watermarking scheme using a scale invariant feature transform (SIFT) matching. According to the technique, a watermark is inserted into a common part between a central image and a synthesized image which is detected through SIFT. Since the technique utilizes a matching scheme, the technique is robust against the DIBR and has a high PSNR. In addition, the technique is greatly robust against JPEG compression and median filtering.
However, since a descriptor separately stored in the SIFT matching process is required, it is difficult to perform blind detection so that the utility is deteriorated.
Therefore, there is a need to provide a watermarking method which is capable of compensating weakness of the methods of the related art.